Firestop systems for pipes and conduits are typically tested to ASTM E 814, Test Method for Fire Tests of Through-penetration Firstops, although other standards exist. This method produces two different ratings: one called the F-rating and the other called the T-rating.
The F-rating is a measure of the minimum amount of time that a design has shown to prevent the propagation of fire from one side of an assembly to the other. The T-rating, on the other hand, is a measurement of the amount of time it takes for the penetrant on the unexposed side to reach 325° F. (163° C.) above ambient at a single thermocouple point or the amount of time that the average of the points reaches greater than 250° F. (121° C.) above ambient. In other words, the test limit has been reached if one thermocouple rises 325° F. (163° C.) above the ambient or if the average rise of all the thermocouples is greater than 250° F. (121° C.) above ambient. The thermocouples are typically placed at critical locations where excessive heat would most likely occur, such as on the penentrant just beyond the assembly and on the firestop sealant. A firestop system must always have an acceptable F-rating equal to or greater than the rating of the assembly that it will be installed in. The T-rating, on the other hand, is a guideline measurement of the temperature performance of the system. For example, a 1-hour rated assembly must have a firestop system with an F-rating of at least one hour, but the T-rating could be zero, depending on the local code requirements.
The inherent heat conductive properties of a penetrant itself have the greatest influence on T-rating performance. Plastic pipes with good insulative properties (e.g. PVC, ABS) achieve high T-ratings, while steel pipes with good conductive properties (e.g. copper, iron, steel, aluminum) achieve very low T-ratings. The low T-rating is usually due to the physical property of the penetrant, not the firestop material. A non-insulated 6″ diameter steel pipe will fail the T-rating in an average of 15 to 30 minutes and copper pipes fail much more quickly. The problem with low T-ratings is also compounded in thin concrete assemblies such as 2½ inch fluted decks. There is not enough concrete in these thinner floor assemblies to thermally draw the heat away from the pipe. Thinner floors make poor heat sinks. The addition of 3″ insulation (e.g. mineral wool, fiberglass) around a metal penetrant will increase the T-rating by as much as two hours. Many through penetration firestop systems require the pipe to be insulated to achieve the high T-rating. There is a severe problem, however, with adding insulation to conduits. During normal operation, conduits tend to heat up from resistance within energized wiring. Insulating the conduits will not allow any excessive heat to escape and increases the chances of an electrical fire. The ampacity rating of the conduit is reduced due to the insulation.